Workers in the art of making and using magnetic recording apparatus for data processing and the like are well aware that the media therefor often present serious wear problems, such as may foreshorten their useful life. For instance, with rigid magnetic recording disc surfaces which support a thin-film magnetic recording head (carried on a "slider") passing at relatively high speeds, the head is all too apt to problematically abrade both itself and the medium.
A related problem is that of undesirably high friction between the disc and head (surfaces). To ameliorate abrasion, workers typically specify that a disc coating exhibit no more than a certain (maximum) coefficient of friction--this also reduces the force necessary to drive the disc past the head. To reduce friction and enhance wear, workers have resorted to various expedients such as "surface-(topical) lubricant" coatings on the disc's recording surface.
This invention is also concerned with such surface lubricants, and with techniques for affixing the lubricants to such record surfaces, preferably by chemical bonding.
Workers recognize the problems using such surface lubricants (on the surface of information carrying media such as magnetic recording discs and tapes). Such lubricants may be applied to a record surface, and held there, by purely mechanical means, for example, by capillary forces. Publications which illustrate physical retention of a lubricant on a solid surface include, for example, U.S. Pat. Nos. 4,188,434; 4,232,072 and 4,268,556. In these patents, no chemical bond is taught between the surface lubricant and the underlying surface.
According to the present invention, a prescribed isocyanate lubricant is applied to a record surface and cured there for a stable firmly-attached, chemically-bonded polymeric lubricant (contact disc with an isocyanate-terminated lube).
Generally "surface lubricants" as here understood are chemically inert and thus exhibit little adherent interaction (such as hydrogen bonding or polar group interaction) with the record surface. Because of such loose adhesion when a record disc, as here understood, is rotated at high speed, the lubricant tends to migrate from the disc (e.g., adhesion forces being gradually overcome by the centrifugal force), and thus the lubricants are spun-off and depleted, with consequent likelihood of "head crash".
According to the present invention, such lube depletion is inhibited by application of a thin film of isocyanate lubricant to the information carrying surface, this film being so chemically-bonded to the surface as to not be readily removed (e.g., by washing or rubbing with isopropanol). Record surfaces apt for this include, for example, magnetic discs, magnetic tapes, sound recording media and the like, especially certain compatible polymeric surfaces.
Such lubricant coatings are particularly apt for polymeric magnetic record disc surfaces (e.g., of the type used in computer recording), especially where the relative head-medium velocity is very high, and the head is highly abrasive (cf. thin film heads) and consequent abrasion and reduced life of both head and media is quite likely. Thus, workers have resorted to various known "surface-lubricant" means for such disc coatings and like media (e.g., see the following U.S. Pat. Nos.: 3,490,946; 3,492,235; 3,523,086; 4,431,702; 3,983,302; 3,837,912; 3,597,273; 4,446,193; also IBM TDB Vol. 26, #7B, p. 3778, December 1983; also article by Budinski in J. Vac. Sci. Technol. Vol. 12, #4, July-August 1975, pp. 786+; and article by Hamada, et al., J. Appl. Phys. 55(6), 15 March 1984, pp. 3775+.
Depletion of surface lubricant on a spinning magnetic recording disc is believed primarily due to "spin-off" initiated by centrifugal forces. Some subsurface lubricant (within the body of the magnetic coating itself) is at times suggested as a potential reservoir for replenishing the surface lubricant so spun-off.
Workers are aware of the "spin-off" problem and recognize that it commonly depletes surface lubricant prematurely inducing head-crash and system failure. A salient purpose of this disclosure is to inhibit such depletion by application and chemical bonding of an isocyanate-terminated (or acid-terminated) lubricant to a polymeric record surface, especially where the polymeric surface exhibits free, reactive groups (or can be induced
so)--e.g., hydroxyl--which can chemically bond with the reactive isocyanate (or acid) groups, to yield urethane or ester bonds.
A related object is to match this isocyanate lube with the characteristics of a lubricant supercoat thereon (e.g., significant mutual solubility and mutual molecular attraction plus substantial inter-diffusion) for improved adhesion thereof and synergistic co-lubrication. Thus, the two lube layers of like composition and structure can, together, act to inhibit depletion of the top lubricant layer.
It is an object of this invention to address such problems and particularly to teach novel magnetic recording compositions including improved surface lubricant systems, especially for systems with rigid disc media and thin film heads.
For instance, compared with a standard ferric oxide computer disc coating in a certain polymeric binder, a novel lube reactive coating system according to our invention applied as a scant undercoat for normal surface lube will reduce spin-off and like depletion, and enhance durability and operating life--to the surprise of workers! (e.g., because our undercoat material is conventionally viewed as a lubricant--thus we are improving adhesion of a lube coat by a lube under-coat!). Such a reactive lube undercoat will include isocyanate-terminations. They also, preferably, include fluorocarbon moieties (preferably fluoro-alkyls such as fluoro-ether polymers); further they are preferably super-coated with a fluorocarbon lube system of similar structure. For instance, a preferred reactive coating comprises a diisocyanate-terminated perfluoro-poly-ether which is very apt for use with a super-coat lube including fluoro-ether constituents (less preferably, the fluoro-version may be substituted for the perfluoropolyether, but will typically be less stable; for instance, where hydrogen replaces fluorine, it will be more likely to react adversely, change properties, offer a point of corrosive attack, etc.)
Now workers have, of course, suggested fluorocarbon surface lubricants for magnetic recording surfaces. But such lubricants, in themselves and in general, are not necessarily effective in affording the kind of superior durability, stability and spin-off resistance we desire; also, in heavy concentrations they can badly impair magnetic performance (e.g., see U.S. Pat. No. 4,431,703).
Some workers have suggested various organic (e.g., organo-fluorine) compounds as lubricants for rigid magnetic recording media (e.g., see U.S. Pat. No. 3,993,846, or U.S. Pat. No. 4,007,314, or U.S. Pat. No. 4,131,717--or see U.S. Pat. No. 4,007,313 mentioning an organo-silicone-fluoride lubricant).
The tendency now is to increase density of recording using very thin magnetic media films having highly polished surfaces and using recording heads which fly practicallly in contact with (if not actually sliding-over) these films. This calls for substantial improvements in surface lubrication for such high-density magnetic media, with lubricant layer specifications becoming very stringent. Such a lubricating layer should be as thin as possible (order of magnitude of several dozen .ANG. or more) and yet be very stable, long-lasting, effective and durable; it must not react with the record-binder lest it plasticize the record surface and, over all, it must not be removed or significantly depleted in use.
Common "migratory" lubricants (which are added to the bulk of the record coating) are now suspect under such circumstances; e.g., their migration rate cannot be controlled with sufficient precision; hence they cannot assure a surface lube-coat which is sufficiently uniform.
"Topic" or surface-applied lubricants (e.g., fluorocarbon oils) are now coming into favor for such media--indeed, in some cases, such as with thin film metallic media, workers feel there is no other conventional way to achieve superior surface lubrication.
As a feature hereof, such media are surface-coated with a scant, well-bonded film: i.e., the (urethane) reaction product of the isocyanate-terminated polymer lubricant and the surface hydroxyls--(or the analogous ester product of the acid-terminated polymer).
Fluorocarbon liquid films are generally known as useful to lubricate magnetic recording media (e.g., rigid discs of the Winchester type; cf. pp. 1073+of IEEE Transactions on Magnetics, Vol. MAG-18, No. 6, November 1982). Such lubricants have desirable properties (oxidative and thermal stability, chemical inertness, wettability, viscosity range, etc.), but the mass of lubricant on the disc must be carefully controlled. If there is too little, head-wear is excessive and "crashes" can occur. If too much lubricant is applied, puddling and "stiction" of the head can occur; e.g., during "start-up". This can result in distortion of a head flexure and disc damage (and so can lead to a latent failure situation; cf. all the factors which cause "stiction" are not known, but excess lubricant seems to be a major contributing factor).